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Three’s a… Lichen?

Perhaps it’s my tendency to root for the underdog, but I love to see researchers challenge long-standing scientific dogma. Some of my favorite underdogs of this variety include Copernicus who determined we live in a heliocentric solar system rather than a geocentric one and Marshall and Warren who discovered that ulcers are caused by Heliobacter pylori rather than stress or diet. More recently, members from John P. McCutcheon’s laboratory discovered that a lichen is composed of three organisms rather than two.

What’s a Lichen?

You’ve probably seen or encountered a lichen before on a trail or in your backyard – they look like colorful flecks of peeling paint found adorning tree trunks worldwide and cover approximately 6% of earth’s surface. You may remember discussing lichens as an example of symbiosis in high school biology. Or maybe you’ve been living under a rock for a while and aren’t familiar with this interesting organism, in which case you might be shocked to discover that a lichen is probably your neighbor.

“I’ve taken a lichen to you!”, says this rock to the lichen on its surface. (photo by S. Rae)

Prevailing Dogma

For a long time lichens were confused as plants or fungi, until the botanist and microscopist Schwendener presented them as a composite of fungus and algae in his dual hypothesis in 1867. Schwendener originally proposed that the fungi exploited the algae, but other scientists showed that the symbiosis was beneficial for both the fungi and the algae – the algae synthesizes carbohydrates for the fungus and the fungus provides nice shelter for the algae to grow. Schwendener’s hypothesis held true upon experimentation and today it is common knowledge that a lichen is composed of a fungus and a photosynthesizing partner (either algae or cyanobacteria). Or so we thought…

For over 150 years people thought lichens arose from the symbiosis of a single fungus and a photosynthesizing partner, but recent research indicates that a lichen is actually a mutualistic relationship between two types of fungus and a photosynthesizing partner. The second fungus went undetected by microscopy for such a long time because it is trapped in a dense area of glycans known as the cortex of the lichen. McCutcheon and colleagues present these findings in a recently published Science report.

Most lichens are comprised of a photosynthesizing organism and a fungal partner of the ascomycota variety. Interestingly, the authors identify the other fungi in the threesome as a fungi of the basidiomycota variety. The two types of fungi mainly differ in their sexual reproductive structures. Ascomycota, like the common baker’s yeast, have little sacs that their spores grow in while basidiomycota spores grow on the end of club-like cells. The authors suggest that the newly identified basidiomycete fungi is responsible for the phenotypic variation in color, shape, and compounds produced among lichens.

Three’s a lichen – lichens are comprised of two fungi and one photosynthesizing partner. (photo by S. Rae)

The findings presented by Spribille et al. represent a shift away from the two-organism paradigm of lichens. Most textbooks students read this year will still present the prevailing paradigm, but in time, as more research confirms this three-organism idea, we may see school curriculum reflect these revelations. Until then, my admiration of scientists who challenge long-standing dogma will simply grow.

Moral of the story: sometimes possible explanations are overlooked for decades because we get stuck thinking about things in a particular way. In Spribille’s case, his lichen specimens might as well have been singing “come and knock on our door, we’ve been waiting for you” because apparently for a lichen, three’s a company too!

If you’re interested in learning more about lichens and fungi, I highly recommend the “Fungi: Friend or Foe” course offered every year at UGA.

Stephanie Halmo is a former middle school science teacher turned graduate student, actively pursuing her Ph.D. in biochemistry from the University of Georgia. In her spare time she likes to dance, volunteer at local schools and tie-dye anything she can get her hands on. She is currently ASO’s News Editor. You can connect with Stephanie on Twitter and Instagram @shalmo or by email: shalmo27@uga.edu